The present invention relates to arrayed waveguide gratings (AWGs), and more particularly to a novel architecture for an AWG.
AWGs, sometimes also known as “phasars” or “phased arrays”, are well known components in the optical communications network industry. An AWG is a planar structure comprising an array of waveguides arranged side-by-side which together act like a diffraction grating. AWGs can be used as multiplexers and as demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-Devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, No. 2, 20 June 1996, and U.S. Pat. No. 5,002,350 and WO97123969.
FIG. 1 illustrates the layout of a conventional AWG. It comprises a substrate (“die”, “chip”) 100 supporting one or more input optical waveguides 110 delivering optical energy into an “input slab” region 112. The slab region is a planar waveguide which confines the input optical energy in only the vertical dimension; the energy is permitted to spread transversely without restriction. The input slab is sometimes referred to herein as an “input free space region”, or an “input free propagation region”.
An image of the input optical energy (or an interference pattern, if there is more than one input optical waveguide) is developed on the far border 114 of the input slab region 112. At this border the light enters the input end 116 of a waveguide array 118 which consists of tens or hundreds of individual waveguides. The array waveguides (not visible individually in FIG. 1) are of lengths which increase linearly across the array, each waveguide having a length which differs from its nearest adjacent waveguide by a value ΔL.
Optical energy exits the waveguide array 116 at an output end 120 thereof, and delivers the light into an “output slab” region 122. Like the input slab, the output slab region is a planar waveguide which confines the input optical energy in only the vertical dimension. The energy is permitted to spread transversely without restriction, and for that reason the output slab is sometimes referred to herein as an “output free space region”, or an “output free propagation region”.
A diffraction pattern is developed on the far border 124 of the output slab region 122, where the light enters a set of one or more output optical waveguides 126. The structure can be used as a demultiplexer if there is only one input waveguide 110 and more than one output waveguide 126; in this case information can be carried on multiple channels (wavelengths) in the single input waveguide and the channels are separated out by the AWG for delivery into the different output waveguides. The structure can also be used as a multiplexer if operated in reverse. It can furthermore be used as a router if there are multiple input waveguides 110 and multiple output waveguide 126.
It is frequently desirable to be able to reduce the physical size of an AWG, either to be able to fit more functions on a chip or to fabricate smaller chips. There are physical limits on how small an AWG can be made, however, since waveguide curves must be made gentle enough to maintain their guiding properties. A significant amount of space is also consumed usually by the input and output waveguides which must fan-in or fan-out to direct the light to or from the edges of the chip. There are also additional minimum-size restrictions imposed by the waveguide-to-waveguide length differential ΔL in the array and the desired number of waveguides in the array, as well as the focal lengths required in the input and output slabs. Other factors may also play a role in thwarting attempts to shrink AWG device designs. It would be desirable if some of these factors could be overcome to enable smaller AWGs.